SIGNATURES OF LANDSLIDE-GENERATED WAVES IN THE SOUTHERNMOST COAST MOUNTAINS, BRITISH COLUMBIA

Landslide-generated displacement waves pose an important, yet underappreciated hazard in many mountainous regions. British Columbia’s steep terrain and extensive shorelines make it susceptible to large displacement waves produced by terrestrial and subaqueous mass movements. However, BC’s relatively short written history and generally sparse population limit documentation of displacement wave events. We are expanding understanding of past landslide-generated waves in BC, and more generally of the types and longevity of landforms and sediments the phenomenon produces. Here, we summarize the geologic and geomorphic signature of the 2007 landslide-generated wave in Chehalis Lake in BC’s southernmost Coast Mountains. We also describe landslide deposits on the floor of nearby Harrison Lake and features along its shoreline that suggest past displacement waves. Vegetation trimlines are the most obvious evidence of these events, but they fade after several decades and disappear within one or two centuries. Impact marks and embedded stones in standing trees at the trimline provide longer-lasting records. Scarps eroded by waves can be detected with LiDAR, even after shorelines revegetate, and are the most persistent of geomorphic indicators. Anomalous sand and gravel layers washed into marshes and other low-energy settings within the area of wave influence may persist even longer than geomorphic indicators. Typically, multiple indicators as well as their spatial and temporal association with well documented landslides are required to confirm past landslide-generated waves. The ages of events are best determined by dating vegetation disturbance and wetland deposits. Our preliminary investigation at Harrison Lake using historical imagery, LiDAR and bathymetric surveys, and field study reveals multiple lines of evidence of landslide-generated waves. We suggest a large, hitherto undocumented wave occurred there in the late 19^th century. This and other events indicated by subaqueous debris fields and subaerial rock falls are currently being investigated. Similar studies elsewhere in BC will undoubtedly reveal evidence of past landslide-generated waves. It is important to prioritize future research in areas with populations and infrastructure potentially at highest risk, for example Harrison Lake.